Immersion Casting

ABSTRACT

A method of immersion casting objects from molten metal, by crystallizing the metal outwardly from a heat-absorbing forming element such that upon removal from the molten metal bath, the solidified object has an internal surface defined by the shape of the forming element, and an outer surface that features random crystallization and a high degree of texture. The method can be facilitated by the interaction of the forming element and molten metal with molten salt provided as a layer on the molten metal. When the object is cast from a high purity metal such as aluminum or copper, the exposed crystal structure is especially random and highly reflective and can be enhanced by electro-chemical brightening.

BACKGROUND

The present invention relates to the casting of decorative metal items, such goblets, vases, or other such vessels, and plaques or other flat substrates. Although such decorative items have been well known for years, if not centuries, artisans continually strive to find new ways of creating interesting shapes, surfaces, and manufacturing techniques that both capture the imagination yet can be achieved in a cost-effective manner.

SUMMARY

The present invention provides a method of casting such metal items or objects from molten metal, by crystallizing the metal outwardly from a forming element such that upon removal from the molten metal bath, the solidified object has an internal surface defined by the shape of the forming element, and an outer surface that features random crystallization and a high degree of texture.

When the object is cast from a high purity metal such as aluminum or copper, the exposed crystal structure is especially random and highly reflective. The brightness of the reflectivity can be enhanced by electro-chemical brightening.

In one general aspect, the invention is directed to a method of manufacturing a shaped metal object, comprising immersing a preform with a shape corresponding to the shape of the metal object into the molten metal bath; maintaining the immersed preform at a temperature lower than the temperature of the molten metal, whereby the molten metal crystallizes outwardly from the preform with increasing thickness of solid metal in a raw shape complementary to the preform; withdrawing the raw shaped solid metal and preform from the molten metal bath together, whereby the shaped solid exhibits a randomly crystallized outer surface; separating the shaped solid from the preform; and preferably treating the randomly crystallized outer surface with a brightening agent to produce a finished shaped metal object.

In another aspect, the invention is directed to a method and associated system of manufacturing a vessel comprising the steps of providing a molten metal bath; immersing a cooled forming element into the metal bath whereby the molten metal crystallizes outwardly from the forming element, thereby creating a solid metal, raw vessel around the forming element; withdrawing the raw vessel and forming element from the molten bath, whereby the raw vessel has developed a randomly crystallized outer surface; and separating the forming element from the raw vessel. As a preferred final processing, the inner surface of the raw vessel can be smoothed and the outer surface can be further brightened and/or sealed.

The interface between the forming element and the outwardly solidifying metal preferably includes an agent that facilitates removal of the forming element and solidified object together from the molten bath, while also facilitating the subsequent separation of the metal object from the perform.

This agent is preferably in the form of a supernatant layer of molten salt over the molten metal. Upon immersion of the forming element into the layer of salt a layer of solid salt solidifies on the forming element. Upon further immersion of the forming element into the molten metal the solidified salt is converted to molten form at the molten metal interface while the salt at the interface with the forming element remains solid. The solid salt at the interface with the forming element shrinks toward and thereby adheres to the forming element and the solidified molten metal at the interface with the molten salt shrinks toward the forming element and thereby adheres to the molten salt. This occurs as the molten metal and salt begin to cool and solidify at the interface. After withdrawing the metal object and forming element together from the molten metal bath, the salt layer can readily be fractured and the forming element slid out of the metal object.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment will be described in detail below with reference to the accompanying drawing, in which:

FIG. 1 is a schematic of one set up for immersion casting of a goblet or the like; and

FIG. 2 is a stylized representation of the finished goblet as observed from above.

DETAILED DESCRIPTION

With reference to FIG. 1, the system set up 10 of one non-limiting example will be described for the casting of a tubular object such as a high purity aluminum vessel. A high-conductivity, e.g., copper, forming element in the shape of a rod or water cooled tube 12 is immersed into a molten metal bath 14 contained in a crucible 16 or furnace on which there is a supernatant layer 18 of molten salt. Upon immersion of the high-conductivity forming element 12 into the bath a layer of solid salt 20 solidifies at the interface with the forming element. The salt can exist in two phases, as a solid at the interface on the forming element and as molten material at the interface between the salt and the molten metal. The salt and molten metal solidify as heat is withdrawn through the forming element 12. However, depending on the salt compound, the salt can remain molten as the metal solidifies, as indicated at 22.

The salt 20 stays in place during this process because it shrinks onto the forming element 12. The aluminum crystals grow from the inside to the outside of the casting 22. For a forming element 12 in the shape of a uniform cylinder, the molten metal solidifies radially outwardly from the axis and downwardly to an extent sufficient to form a base 22′ for the vessel.

After a predetermined or monitored dwell time, sufficient aluminum has crystallized 22 on the immersed forming element 12 to establish the dimensions of the vessel. The forming element 12 with attached metal vessel 22 is extracted from the crucible 16. After cooling, the casting 22 is removed by slight tapping of the forming element. The salt layer 20 fractures and the casting is simply pulled off the forming element. If further efforts are required to remove the casting from the forming element, water is sprayed on the casting to dissolve the salt.

With further reference to the schematic representation of FIG. 2, the raw casting 24 has an inner sidewall diameter 26 and associated inner surface texture established by the interface with the salt. This surface in not normally visible in the finished vessel, but can optionally be smoothed with a rotary sander or the like. The raw vessel is cleaned before final processing.

The raw casting 24 has an unusual, highly textured outer surface 28. This is shown schematically as an irregular or random pattern of peaks and valleys of crystalline structure, of randomly varying height and depth. The bottom of the casting and top surface 30 can be ground smooth, with optional tapering at the top for a goblet or the like.

For enhancing the decorative appeal, the outer surface 28 can be finished with the following steps: (a) electrochemical brightening in a standard electro-chemical brightening bath containing, e.g., phosphoric acid; (b) electrolytic anodizing in a solution of sulfuric acid; (c) dyeing in a solution of ferric ammonium oxalate; and (d) sealing in a high temperature water bath of at least 200-212 degrees F. with additions of nickel salts.

In a working example of a 99.99% pure molten aluminum bath, a solid cooper forming element with associated heat sink, and a sodium chloride salt, the following table shows the relationship of densities and melt temperatures:

Al Cu NaCl Spec. Gravity 2.70 8.92 2.16 Melting Point (C.) 660 1083 801

In general, the metal material has a relatively lower melt temperature and a relatively higher density than the salt, and the forming element has a higher melt temperature than the metal and the salt. However, regardless of the melt temperature of the salt, the density of the salt must be lower than the density of the metal to assure that the salt layer floats on the molten metal. The temperature of the molten metal should be high enough for the salt to be molten at the interface with the molten bath as the forming element enters the bath, but low enough that the salt solidifies at the interface with the forming element as the forming element enters the bath and the molten metal solidifies outwardly from the forming element during the dwell time of the forming element as the forming element provides a heat sink.

The use of dual molten salt layers for casting a metal or other sheet between them is described in my U.S. Pat. No. 2,754,550, issued Jul. 17, 1956 for “Method for the Casting of Sheets of a Fusible Material”, the disclosure of which is hereby incorporated by reference. Although the immersion casting method of the present invention is different and not readily derivable from my prior patent, many examples of the salts and metals listed therein are usable in the present invention. Barium chloride alone or mixed with sodium chloride is another good candidate for the salt layer of the present invention, especially for the casting of a steel object. Other options include casting of tin, with a sodium chloride layer and copper rod; casting of silver, with a silicon oxide layer and an iron rod. Graphite can also serve as a suitable rod.

With further reference to FIG. 1, depending on the size and complexity of the metal object to be cast, the forming element 12 can be a solid rod for immersion into the bath, with a large, integrated disc 32 or the like at the upper end of the rod providing a relatively large heat sink from the rod. A drive mechanism with associated struts 34 or the like and controller (not shown) raise and lower the integrated rod and disc together as shown at 36, into and out of the molten bath 14, according to the desired shape of the metal object 22.

For example, a goblet having a cylindrical sidewall can be formed by simply immersing a cylindrical forming element into the molten metal bath, leaving the forming element in position within the bath for a preselected time, and then removing the forming element with solidified object.

For a vase, the forming element can also be a cylindrical rod for which the dwell time along the axial dimension of the rod is varied to produce a varying outer diameter of the solidified metal. Alternatively, the forming element can have a non-uniform diameter and remain fully immersed in the bath for a specified period of time, which will produce a vessel having a varying inside diameter and varying outside diameter.

It can be appreciated that a wide variety of forming elements 12 can be used for implementing the inventive concept. The forming element can be a thin plate defining opposite planar faces. FIG. 1 can represent this as well, wherein the one edge of a plate-shaped forming element 12 is visible, with the length of the plate 12 extending well into the plane of the drawing sheet, and the relative thickness of the casting 22 less than suggested in the figure. The forming element can be immersed in a molten bath to produce a solid metal layer all around the forming element, but with the predominant feature being solidification of two relatively large metal plates which exhibit an unusual, highly textured crystalline surface. Upon removal from the molten bath, the forming element can be separated from what in essence is a metal sleeve. The connecting web (thickness edges) between the planar faces can be cut and ground to produce two beautiful plates that can be further processed into engraved plaques or frames for mounting any form of graphic or the like.

In a more complex but efficient implementation for high production volume, the forming element 12 can be actively cooled with internal cooling flow paths 38 a, 38 b connected to a cooling manifold 32. The cooling manifold 32 is connected to an inlet 40 from a source of cooling fluid, and an outlet 42 for return flow to be re-cooled. The manifold has internal piping or baffling 44 a, 44 b for providing the cooling flow 38 a, 38 b within the forming element tube 12. The manifold 32 and forming element 12 are vertically displaceable 36 with respect to the crucible 16.

To achieve the highly textured outer surface, the object 22 must be withdrawn from the molten bath 14 while the metal material adjacent the side and bottom walls 46, 48 of the crucible remains molten. This assures that the outward crystallization is not inhibited by the sidewalls of the crucible. With freedom for uninhibited outward growth, the crystal structure at the outer surface of the raw, solidified metal object exhibits a preferred peak-to-valley roughness in the range of about 0.125 to 0.250 inch. 

1. A method of manufacturing a shaped metal object, comprising: providing a molten metal bath contained in a heated crucible; immersing a cooling element into the metal bath whereby the molten metal crystallizes radially outwardly from the cooling element, forming a solid, shaped metal object around the cooling element; withdrawing the shaped metal object and cooling element from the crucible, whereby the shaped metal object has a randomly crystallized outer surface; and withdrawing the cooling element from the shaped metal object.
 2. The method of claim 1, wherein a supernatant layer of molten salt is present over the molten metal; the cooling element is immersed into the layer of salt whereby a layer of salt solidifies at an interface on the cooling element; and upon further immersion of the cooling element into the molten metal, solidified salt at interface with the molten metal is initially converted to molten form while the salt at the interface with the cooling element remains solid.
 3. The method of claim 2, wherein the solid salt at the interface with the cooling element shrinks toward and thereby adheres to the cooling element; and the solidified molten metal at the interface with the molten salt shrinks toward the cooling element and thereby adheres to the molten salt.
 4. The method of claim 3, wherein after withdrawal of the metal object and cooling element from the crucible the salt exists as a solid layer between the cooling element and a solid raw vessel; the salt layer is fractured or dissolved; and the cooling element slides out of the metal object.
 5. A method of manufacturing a shaped metal object, comprising: providing a molten metal bath; providing a preform with a shape corresponding to the shape of the metal object; immersing the preform into the metal bath; maintaining the immersed preform at a temperature lower than the temperature of the molten metal, whereby the molten metal crystallizes outwardly from the preform with increasing thickness of solid metal in a raw shape complimentary to the preform; withdrawing the raw shaped solid metal and preform from the molten metal bath together, whereby the withdrawn raw shaped solid metal exhibits a randomly crystallized outer surface; separating the raw shaped metal solid from the preform; and treating the randomly crystallized outer surface with a brightening agent to produce a finished shaped metal object.
 6. The method of claim 5, wherein the preform is cylindrical.
 7. The method of claim 5, wherein the preform is a plate.
 8. The method of claim 5, wherein an interface agent is provided between the immersed preform and the molten metal, which interface agent adheres to both the preform and metal surrounding the preform such that the raw shaped solid metal and preform can be withdrawn from the molten metal bath together; and the raw shaped metal solid and preform are separated by disturbing the interface agent.
 9. The method of claim 8, wherein the withdrawn preform and adhered solid metal are cooled before being separated; and the cooling reduces the adhesion of the interface agent such that the preform can be'separated from the metal solid.
 10. The method of claim 8, wherein the interface agent is provided as a layer of salt on the molten metal bath.
 11. The method of claim 5, including supplying a flow of cooling fluid within the preform while the preform is immersed in the molten metal.
 12. The method of claim 5, including displacing the preform into and out of the molten bath at a predetermined variable rate.
 13. A method of manufacturing a shaped metal object, comprising: selecting a crucible having a working chamber; depositing a metal material into the crucible, which metal material has a relatively low melt temperature and a relatively high density; introducing a salt into the crucible, which salt has a relatively lower density than the density of the metal; heating the crucible to liquefy the metal and a supernatant layer of salt; passing a forward portion of a preform into the heated crucible, through the layer of salt into the molten metal, whereby the salt adheres to the forward portion of the preform before the forward portion of the preform enters the molten metal; connecting the preform to a heat sink whereby the molten metal around the salt on the preform cools below the melt temperature of the metal, and the metal crystallizes radially outwardly from the preform with a solid metal thickness around the preform that is dependent on the length of time the preform is in the molten metal; lifting the preform with adhered solidified metal and salt out of the crucible; removing the preform from the solidified metal, leaving a raw metal object with a randomly textured outer surface.
 14. The method of claim 13, wherein the preform is solid and the heat sink is a solid body integral with and extending transversely to the preform above the crucible.
 15. The method of claim 13, wherein the metal is one of aluminum or tin, the salt is sodium chloride, and the preform is copper.
 16. The method of claim 13, wherein the salt comprises at least one or combination of sodium chloride and barium chloride.
 17. The method of claim 13, wherein the metal is silver, the salt is silicon oxide and the rod is iron.
 18. The method of claim 13, wherein the preform is graphite.
 19. The method of claim 13, including cleaning and brightening the textured outer surface of the raw vessel.
 20. The method of claim 19, wherein the textures outer surface is electrochemically brightened.
 21. The method of claim 20, wherein the electrochemically brightened surface is sealed with a nickel salt bath.
 22. The method of claim 13, wherein the salt has a relatively higher melt temperature than the melt temperature of the metal and all the salt on the immersed rod solidifies as the molten metal cools outwardly.
 23. The method of claim 13, wherein the salt has an interface with the molten metal and the salt at the interface remains molten as the molten metal solidifies outwardly. 